As semiconductor technology moves toward ever faster and smaller devices, new challenges arise. In particular, when contemplating devices that require shallow (<9 nm) defect-free extensions, conventional techniques fall short. Cluster implantation, plasma immersion ion implantation and solid phase epitaxial regrowth are known for sub-micron device fabrication, but their use to make nanoscale devices is problematic. Monolayer doping is attractive, but the known means for providing a monolayer are not. Solution passivation with ammonium sulfide on binary and ternary semiconductors is known, but the procedures described in the literature would require the use and disposal of industrial quantities of 20% aqueous (NH4)2Sx. Ammonium polysulfide and its precursor, ammonium sulfide, are toxic; both storage and disposal raise safety issues. Twenty percent aqueous ammonium sulfide and polysulfide must be stored under the most stringent conditions and, after use, must be disposed of by collection and off-site decontamination. Contact with acidifying substances in wastewater generates hydrogen sulfide, which is itself volatile, offensive and toxic. There is a general need for a process for producing a sulfur monolayer on a semiconductor, which process is practical on an industrial scale.
Although the use of solutions of ammonium polysulfide as low as 0.2% has been reported for passivating the surface of InAs nanowires to fabricate an ohmic contact [Suyatin et al. Nanotechnology 18, 105307 (2007)], in general, systematic studies of ammonium sulfide passivation of InGaAs [O'Connor et al. J. Appl. Phys. 109, 024101 (2011)] have suggested that the optimal concentration for that purpose was 10%. Similarly, U.S. Pat. No. 6,924,218 describes a 2:9 mixture of 20% ammonium sulfide and 30% aqueous ammonia for passivating the surface of an InAlAs Schlottky layer.